1. The Field of the Invention
The present invention relates generally to pumping control systems. More specifically, the invention relates to systems for controlling dispensing of medications to more accurately control dispensing of medication.
2. The Relevant Art
Medical science often requires that liquids be administered to a patient in a variety of situations. These liquids include simple intravenous feeding solutions, saline solutions for providing pressure to the eye during ocular surgery, contrast media infused to enhance imaging abilities, blood administered during transfusions, and nutrient solutions, medications, chemotherapy solutions, or the like delivered via intervenal or enteral means. In virtually all these applications, reliable and precise delivery of liquids is critical to successful treatment of the patient. In some applications, improper delivery of the liquids such as overrun may be life threatening to the patient.
Systems that are reliable and precise are often costly to design, manufacture and test. Indeed, the relative cost of medical care continues to increase often due to the increased cost of medical technology. Accordingly, within the health care industry there is a high need for low-complexity technology that is effective and reliable.
Overrun is particularly dangerous in medical applications in that even when detected, excessive delivery of a liquid often cannot be reversed. Overrun may be caused by hardware failures such as electronic switches being stuck in a certain state. Controller failures are also a problem in that processors are particular sensitive to environmental factors such as temperature and static. The firmware associated with a controller may contain logic errors or bad memory cells resulting in runaway programs that may xe2x80x9ccrashxe2x80x9d. Runaway or crashing programs may leave control circuitry stuck in a certain state such as in a pumping state where liquid is being delivered at a high rate.
In addition to reliability to prevent overrun and other errors associated with component failure, liquid dispensing systems need to be precise. Precise delivery facilitates the adjustment of dosages, to rates and levels that are optimum for treatment of the patient. Precision also facilitates consistency over time and between various devices and systems, a highly desirable feature in dosage systems.
Stepper motors are often used in applications that require precision. Unfortunately stepper motors require complex control signals that must be properly phased to advance the motor. Stepper motors are also often costly and difficult to test.
Standard motors such as induction motors are typically low cost but suffer from lack of precision in that motors often freewheel after power is cut off. Freewheeling results in liquid overage and overage variability in that the duration and speed of freewheeling is affected by environmental and usage factors such as temperature and motor speed.
Accordingly, what is needed is a liquid dispensing control system for standard motors that is low-cost, and reliable, and that eliminates freewheeling and prevents overrun in the event of system failures.
The apparatus of the present invention has been developed in response to the present state of the art, and in particular, in response to the problems and needs in the art that have not yet been fully solved by currently available liquid dispensing systems. Accordingly, it is an overall object of the present invention to provide an improved method, apparatus and system for dispensing liquids that overcomes many or all of the above-discussed shortcomings in the art.
In particular, it is an object of the present invention to provide a control system for a liquid dispensing system that is low-cost, reliable, eliminates or reduces freewheeling, and substantially prevents overrun in the event of system failures.
These and other objects of the invention are realized in a control system for a liquid dispensing system which includes an electronic brake to stop motor freewheeling, power and ground switches to reliably control the motor, and a watchdog circuit to monitor the controller and disable the motor in the event of system failures.
In accordance with a first aspect of the invention, the electronic brake grounds a power terminal of the motor in response to a stop signal. Grounding the power terminal creates a braking effect as kinetic energy from mechanical inertia is converted to electromagnetic energy within the windings of the motor. The brake in turn absorbs the electromagnetic energy to greatly reduce or eliminate freewheeling. In essence, the motor momentarily acts as a generator allowing the brake to quickly absorb and stop the kinetic energy present in the dosage delivery system.
In accordance with a second aspect of the invention, redundant switches, one to connect to power and the other to connect to ground, ensure that the failure of a switch does not result in the motor being stuck in a running state.
In accordance with a third aspect of the invention, a watchdog circuit monitors a controller heartbeat signal to ensure that the controller is reliably providing a valid xe2x80x98beatxe2x80x99 at an acceptable rate. If so, the watchdog asserts an enable signal which activates one of the switches that control power to the motor. If the required conditions are not met, the watchdog de-asserts the enable signal placing a switch in a non-conductive state, thus disabling the motor and preventing the possibility of overrun.
In accordance with a fourth aspect of the invention, a controller starts or ends each dispensing cycle with a diagnostic which determines whether the liquid dispensing system is fully operational. If not, the controller may perform shutdown operations such as disabling the motor along with error indication operations such as emitting an alarm tone and displaying an alarm message.
To achieve the foregoing objects, and in accordance with the invention as embodied and broadly described herein in the preferred embodiments, an apparatus, method and system for delivery of liquids is described that is reliable, precise and low-cost.